Our long-term goal is to understand the mechanisms by which environmental signals regulate circadian oscillators as well as how circadian oscillators are coupled to each other, using neurons in the rodent suprachiasmatic nucleus (SCN) as a model system. A variety of evidence suggests that glutamate plays a critical role in the transmission of photic information from the environment to the SCN and that the peptide PACAP is a co-transmitter with glutamate at the RHT/SCN synaptic connection. A major goal of this application is to understand the mechanisms by which PACAP modulates glutamate-induced signaling in SCN neurons. We will present data indicating that PACAP enhances AMPA currents in SCN neurons from mice. Furthermore, data from a new line of transgenic animals in which the PACAP gene has been inactivated demonstrate a reduction in the magnitude of the effects of light on the circadian system. Thus, we feel confident that both glutamate and PACAP play a role, be it as yet undetermined, in mediating the effects of light on the circadian system. Many of the retino-recipient SCN neurons receiving this photic information themselves express the peptide VIP as well as GABA. These SCN cells synapse largely onto other SCN cells and presumably use these molecules to communicate photic information to other cells in the SCN. Thus, a second goal is to understand the mechanisms by which VIP modulates GABA-induced signaling pathways in SCN neurons. For this project, we will present data consistent with our hypothesis that VIP acts to modulate GABA currents in SCN neurons in mice. Encouragingly, data from a new line of transgenic animals in which the VIP gene has been inactivated demonstrate major disruptions in the circadian system that are consistent with VIP's role as a coupling agent within the SCN. By carrying out this research, we will address important questions about how SCN cells are coupled to the environment as well as how SCN cells are coupled to each other. In addition, we hope to use the SCN as model system to better understand the role of peptide co-transmitters in mediating cell-to-cell communication. These questions will be addressed using electrophysiological and calcium imaging techniques on SCN neurons in a mouse brain slice preparation. In addition, transgenic mice lacking PACAP and VIP will be analyzed with behavioral and anatomical tools. These newly developed mice are likely to prove a useful tool for circadian rhythms research.